1. Field of the Invention
The present invention relates to an electrical device characterization method and apparatus, and more particularly, to an electrical device characterization method and apparatus using a plurality of buffers for voltage clamping and minimizing leakage currents.
2. Description of the Prior Art
Semi-conductor products, such as mobile phones, computers, etc., find more and more applications as the technology advances, and microelectronic integrated circuits (ICs) are considered as one of the most significant semi-conductor elements. As the semi-conductor process scaling evolves with technology, process variation becomes much more crucial and hard to manipulate. For example, in sub-micron technology, lithographic variation, dopant variation, corner variation, temperature, etc., those minor variations all have great influence on device characterization.
Therefore, it is necessary to improve yield rate and verification process of electrical device fabricated in sub-micron semi-conductor processes. First of all, all electrical devices, for example, metal-oxide-semiconductor field effect transistors (MOSFETs), must be quickly examined and characterized, for example, the turn-on current and threshold voltages of the MOSFETs must be examined quickly and accurately to fulfill massive market requirements. To speed up electrical device characterization, those electrical devices are all lumped together into an array and tested automatically via a programmed computer system. However, since all devices are concentrated into an area, it is hard to avoid leakage current flowing through non-selected electrical devices, and those leakage currents would degrade the characterization result.
To alleviate the leakage current issue, some prior arts introduce on-chip circuitry to clamp voltages at drain terminals and gate terminals of an array of MOSFETs 1001, please refer to FIG. 1, which is a schematic diagram of a test structure 1000 of conventional characterization technology. The test structure 1000 in FIG. 1 utilizes test units 1100a, 1100b, 1100c, and 1100d for voltage driving, clamping or sensing at drain terminals and gate terminals of the MOSFETs 1001, and source terminals of the MOSFETs 1001 are sensed, sank and measured by MOSFET 1200a in conjunction with MOSFETs 1200b, 1200c and 1200d, respectively. The test units 1100a, 1100b, 1100c, and 1100d can be implemented by a plurality of MOSFET switches with multiplexing function, further details are omitted for brevity. By making use of those test units, a selected device under test (DUT) e.g., the MOSFETs 1001, can be fully driven while the gate terminals or the drain terminals of the other non-selected electrical devices are clamped with external voltage sources, and with the voltage clamping function, the leakage currents in the non-selected devices can be decreased, and so as the undesirable voltage drops appearing along the measuring routing lines. However, the test structure 1000 shown in FIG. 1 still requires a significant amount of time to measure a turn-on current and the threshold voltage of the DUT, i.e., the MOSFET 1001, since the voltages at the drain terminal and the gate terminal of the MOSFET 1001 must be calibrated via several iteration, in addition, the clamping voltages being forced to the non-selected devices are supplied externally, the gate terminals or the drain terminals of the non-selected electrical devices may be supplied with improper voltages leading to an incomplete isolation, and as a result, the clamping function may not be able to work as expected, the leakage current issue may still deteriorate the electrical device characterization results. Besides, the test structure 1000 shown in FIG. 1 is not suitable to integrated electrical devices of different dimensions into a same test region, when a DUT is being characterized, the gate terminals or the drain terminals of the non-selected electrical devices of different dimensions are clamped by a same external voltage; since electrical devices of different dimensions requires different turn-off condition, a single external voltage is insufficient to prevent leakage currents flowing through the non-selected electrical devices of different dimensions.
Therefore, traditional device characterization mechanism requires to be improved in consideration of leakage currents, characterization speed and measurement accuracy.